It is well known to provide electronic article surveillance systems to prevent or deter theft of merchandise from retail establishments. In a typical system, markers designed to interact with an electromagnetic or magnetic field placed at the store exit are secured to articles of merchandise. If a marker is brought into the field or "interrogation zone", the presence of the marker is detected and an alarm is generated. Some markers of this type are intended to be removed at the checkout counter upon payment for the merchandise. Other types of markers are deactivated upon checkout by a deactivation device which changes an electromagnetic or magnetic characteristic of the marker so that the marker will no longer be detectable at the interrogation zone.
One type of magnetic EAS system is referred to as a harmonic system because it is based on the principle that a magnetic material passing through an electromagnetic field having a selected frequency disturbs the field and produces harmonic perturbations of the selected frequency. The detection system is tuned to recognize certain harmonic frequencies and, if present, causes an alarm.
Another type of EAS system employs magnetomechanical markers that include a magnetostrictive element. For example, U.S. Pat. No. 4,510,489, issued to Anderson et al., discloses a marker formed of a ribbon-shaped length of a magnetostrictive amorphous material contained in an elongated housing in proximity to a biasing magnetic element. The magnetostrictive element is fabricated such that it is resonant at a predetermined frequency when the biasing element has been magnetized to a certain level. At the interrogation zone, a suitable oscillator provides an AC magnetic field at the predetermined frequency, and the marker mechanically resonates at this frequency upon exposure to the field when the biasing element has been magnetized to a certain level.
According to one technique disclosed in the Anderson et al. patent, the marker has, in addition to the aforesaid resonant frequency, an "anti-resonant frequency" at which the stored mechanical energy resulting from magneto-mechanical coupling is near zero. An interrogation circuit which provides the magnetic field at the interrogation zone is swept through a frequency range that includes the marker's resonant and anti-resonant frequencies, and receiving circuitry is provided at the interrogation zone to detect the marker's characteristic signature by detecting a peak transmitted energy level which occurs at the resonant frequency, and a valley level at the anti-resonant frequency.
Anderson et al. also propose that the magnetostrictive element be subjected to annealing in the presence of a magnetic field to enhance a magneto-mechanical coupling factor k. According to Anderson et al., a larger coupling factor k increases the detectability of the marker's characteristic signature.
In still another surveillance system proposed by Anderson et al., a magnetostrictive marker is used with an interrogation frequency that is not swept, but rather remains at the marker's resonant frequency. The interrogation field at this frequency is provided in pulses or bursts. A marker present in the interrogation field is excited by each burst, and after each burst is over, the marker undergoes a damped mechanical oscillation. The resulting signal radiated by the marker is detected by detecting circuitry which is synchronized with the interrogation circuit and arranged to be active during the quiet periods after bursts. EAS systems of this pulsed-field type are sold by the assignee of this application under the brand name "Ultra*Max" and are in widespread use.
For markers used in pulsed-interrogation systems, the amplitude and duration of oscillations which the magnetostrictive element continues to exhibit after the end of each excitation pulse are very important. The greater the amplitude and duration of the residual oscillations (known as "ring down"), the more unique is the signal during the quiet period in the interrogating zone, and therefore the easier it is for the marker to be detected by the detecting circuitry.
In order to provide the desired ring down amplitude and duration, it is advantageous that the magnetomechanical marker exhibit a quality factor (referred to as "Q") in the range of 250 to 300. The quality factor Q varies inversely with the bandwidth of the marker, and it is therefore desirable that the marker have a very narrow bandwidth. However, because of the narrowness of the bandwidth around the marker's resonant frequency, the ring down amplitude is very much adversely affected if the resonant frequency of the marker deviates from the frequency of the interrogation field.
According to a conventional technique for fabricating magnetomechanical markers, the magnetostrictive element is formed by cutting a strip from a long ribbon-shaped casting of an amorphous material known as Metglas.RTM. 2826MB (which has a composition of Fe.sub.40 Ni.sub.38 Mo.sub.4 B.sub.18). Each strip is then mounted in a housing together with a semi-hard magnet which has been magnetized to saturation to provide a bias field for the magnetostrictive element.
It has been found that the as-cast Metglas ribbon exhibits variations in material composition along the length of the ribbon so that respective strips cut along the length exhibit different magnetostrictive properties. The variation in the resulting magnetostrictive elements is so great that, according to one manufacturing process, it is necessary to measure the resonant frequency of each strip. If required, the length to which each strip is cut, after the third strip of a batch, is adjusted based on the measured resonant frequencies of the previous three strips. In general, the cut length must be adjusted often, sometimes for every strip, and generally after no more than five or six strips. Thus, to compensate for the variation in the conventional as-cast material, the conventional process for manufacturing magnetostrictive elements includes frequent testing of the resonant frequency of the cut strips, and then adjusting the length to which the strips must be cut to obtain the desired resonant frequency. This process is both labor intensive and time consuming. Also, the production yield resulting from this process is less than optimal, since the necessary adjustments in the cut length can not always be predicted with sufficient accuracy to make up for the variations in material. As a result, some of the markers produced by this process fail to have the desired resonant frequency and must be discarded.
There are other factors which may also cause the resonant frequency of a marker to deviate from the desired frequency. For example, the marker's resonant frequency may be shifted from the desired frequency because the strength of the biasing field provided by the biasing element deviates from a standard level. The deviation of the biasing field may be due to variations in the size (e.g. thickness) or composition of the biasing element, or the method of processing used in forming the biasing element. The shift in the marker's resonant frequency because of deviation in the biasing field cannot be detected until after the marker has been assembled, and thus cannot be compensated for by adjusting the length of the magnetostrictive element, which is formed by a cutting operation prior to assembly of the marker.
Another matter of concern is deactivation of the marker. Deactivation of magnetomechanical markers is typically performed by degaussing the biasing element so that the magnetostrictive element ceases to be mechanically resonant or its resonant frequency is changed. However, when the biasing element is degaussed, although the marker is no longer detectable in a magnetomechanical surveillance system, the magnetostrictive element may nevertheless act as an amorphous magnetic element which can still produce harmonic frequencies in response to an electromagnetic interrogating field. This is undesirable because after a purchaser of an item bearing the magnetomechanical marker has had the marker degaussed at the checkout counter, that purchaser may then enter another retail shop where a harmonic EAS system may be in use and where it would be possible for the degaussed marker to set off an alarm because it may generate harmonic frequencies in response to an interrogation signal in the second retail store.